1. Field of the Invention
The present invention relates to an ultrasonic motor capable of producing a high output with a low drive voltage and to a method of fabricating the ultrasonic motor.
2. Description of the Related Art
In recent years, the demand for smaller and lighter parts for automotive vehicles has risen. Especially, demand is high for a quiet motor, for use with automotive vehicles, which is not only capable of producing a high output at a low speed but also has superior response and control characteristics.
Under such a situation, it has been researched to change the motor for automotive vehicles from a DC motor requiring a large, heavy reduction gear to an ultrasonic motor using a piezoelectric device requiring no reduction gear.
The piezoelectric device used in the prior art, however, only has a low output drive (about 1Nxc2x7m for 150 Vrms) and therefore the conventional ultrasonic motor does not have a high output.
In view of the problem mentioned above, the object of the present invention is to provide an ultrasonic motor capable of producing a high output and a method of fabricating the ultrasonic motor.
According to one aspect of the invention, there is provided an ultrasonic motor comprising an annular piezoelectric device, a transducer arranged on the piezoelectric device, a rotary member in pressure contact with the transducer, and means for applying two different types of high frequency AC voltages about 90 degrees apart in phase from each other to the piezoelectric device thereby to generate a traveling wave in the transducer and thus to frictionally drive the rotary member, wherein the product of the piezoelectric constant d31 constituting the piezoelectric characteristic obtained by the resonance-antiresonance method for the drive source including the piezoelectric device and the transducer in close contact with each other, the admittance Ym and the mechanical quality factor Qm is d31xc3x97Ymxc3x97Qmxe2x89xa763xc3x9710xe2x88x9210 mxc2x7S/V.
In the first aspect of the invention, a drive source is employed in which the product of the piezoelectric constant d31 constituting the piezoelectric characteristic obtained by the resonance-antiresonance method of the drive source including the piezoelectric device and the transducer in close contact with each other, the admittance Ym and the mechanical quality factor Qm, i.e. d31xc3x97Ymxc3x97Qmxe2x89xa763xc3x9710xe2x88x9210 mxc2x7S/V, so that a high-output ultrasonic motor can be provided.
According to a second aspect of the invention, there is provided an ultrasonic motor in which the evaluation of the piezoelectric characteristic according to the resonance-antiresonance method for the drive source with the piezoelectric device and the transducer in close contact with each other is preferably conducted by applying a sine wave of xc2x11 V within a frequency range containing the resonance-antiresonance frequency of the motor in one of the two phases in which a high-frequency AC voltage is applied for driving the ultrasonic motor.
Under this condition, a drive source is employed in which the product of the piezoelectric constant d31 obtained by measurement, the admittance Ym and the mechanical quality factor Qm, i.e. Qm d31xc3x97Ymxc3x97Qmxe2x89xa763xc3x9710xe2x88x9210 mxc2x7S/V, thereby making it possible to provide a high-output ultrasonic motor more positively.
According to a third aspect of the invention, there is provided an ultrasonic motor further comprising an adhesive layer 2 to 10 xcexcm thick inserted between the piezoelectric device and the transducer constituting the drive source.
In the case where the thickness of the adhesive layer is smaller than 2 xcexcm, the bonding strength between the piezoelectric device and the transducer is so low that the durability is insufficient.
In the case where the thickness of the adhesive layer is larger than 10 xcexcm, on the other hand, the dielectric constant sharply drops with the piezoelectric constant and the transducer in close contact with each other, and at least a portion of the electrodes formed on the surface of the piezoelectric device is not in contact with the transducer made of a metal part, with the result that the maximum torque of the motor is not sufficiently large.
According to a fourth aspect of the invention, there is provided an ultrasonic motor comprising an annular piezoelectric device, a transducer arranged on the piezoelectric device, a rotary member in pressure contact with the transducer, and means for applying two different types of high frequency AC voltages about 90 degrees apart in phase from each other to the piezoelectric device thereby to generate a traveling wave in the transducer and thus to frictionally drive the rotary member, wherein the composition of the piezoelectric device is given by a general formula
Pbaxe2x88x92bSRb{(ZrxTi1xe2x88x92x)1xe2x88x92yxe2x88x92zMnySbz}O3
where a=0.90 to 0.97, b=0.03 to 0.07, x=0.48 to 0.53, y=0.005 to 0.02 and z=0.01 to 0.04.
The ultrasonic motor using the piezoelectric device described above can produce a higher output than the conventional ultrasonic motor.
According to a fifth aspect of the invention, there is provided a method of fabricating an ultrasonic motor comprising an annular piezoelectric device, a transducer arranged on the piezoelectric device, a rotary member in pressure contact with the transducer, and means for applying two different types of high frequency AC voltages about 90 degrees apart in phase from each other to the piezoelectric device thereby to generate a traveling wave in the transducer and thus to frictionally drive the rotary member, comprising the steps of measuring the resonance-antiresonance characteristic of the impedance by applying the sine wave of xc2x11 V to a drive source including the piezoelectric device and the transducer in close contact with each other in the frequency range containing the resonance-antiresonance frequency of the motor in one of the two phases of the applied high-frequency AC voltage, measuring the piezoelectric constant d31, the admittance Ym and the mechanical quality factor Qm obtained from the resonance-antiresonance characteristic, and selecting a drive source in such a manner that the product of the piezoelectric constant d31, the admittance Ym and the mechanical quality factor Qm, i.e. d31xc3x97Ymxc3x97Qmxe2x89xa763xc3x9710xe2x88x9210 mxc2x7S/V.
By employing this fabrication method, it can be decided what kind of drive source is to be used for obtaining an ultrasonic motor capable of producing a high output, and therefore such a high-output ultrasonic motor can be easily fabricated.
According to a sixth aspect of the invention, there is provided a method of fabricating an ultrasonic motor further comprising an adhesive layer 2 to 10 xcexcm thick interposed between the piezoelectric device and the transducer making up the drive source.